feat: 结构化 observation 替代 statusDetail，API 层动态构造 detail

- CheckResult: statusDetail -> observation (持久化) + detail (API 动态派生)
- 存储: status_detail 列 -> observation TEXT (JSON)
- CheckerDefinition: 新增 buildDetail(observation) 方法
- 各 checker 返回结构化 observation，API 层通过 registry 调用 buildDetail
- HTTP: bodyPreview 在 status/header 失败时也提前采集
- UDP: observation 包含 durationMs，未响应归为 error failure
- CMD: 超时/输出超限时保留已收集 observation
- TCP: connectTimeMs 仅含连接建立耗时，不含 banner 等待
- 新增 buildDetail 单测和 mapCheckResult 覆盖测试
- 同步 openspec 主规范，归档 checker-observation 变更

tests/server/checker/engine.test.ts

@@ -90,7 +90,7 @@ describe("ProbeEngine", () => {
     const results = (mockStore as unknown as { _results: Array<Record<string, unknown>> })._results;
     expect(results.length).toBe(1);
     expect(results[0]!["matched"]).toBe(true);
-    expect(results[0]!["statusDetail"]).toBe("exitCode=0");
+    expect((results[0]!["observation"] as Record<string, unknown>)["exitCode"]).toBe(0);
   });
 
   test("多个目标并发执行", async () => {
@@ -181,7 +181,7 @@ describe("ProbeEngine", () => {
       expect(results[0]!["targetId"]).toBe("reject-cmd");
       expect(results[0]!["matched"]).toBe(false);
       expect(results[0]!["durationMs"]).toBeNull();
-      expect(results[0]!["statusDetail"]).toBeNull();
+      expect(results[0]!["observation"]).toBeNull();
       expect(results[0]!["failure"]).toEqual({
         kind: "error",
         message: "boom",
@@ -288,7 +288,7 @@ describe("ProbeEngine", () => {
       const results = (mockStore as unknown as { _results: Array<Record<string, unknown>> })._results;
       expect(results.length).toBe(1);
       expect(results[0]!["matched"]).toBe(true);
-      expect(results[0]!["statusDetail"]).toBe("HTTP 200");
+      expect((results[0]!["observation"] as Record<string, unknown>)["statusCode"]).toBe(200);
     } finally {
       void httpServer.stop();
     }